Using an assay which specifically measures the number of RNA polymerase initiation sites, we are attempting to identify the factors which control RNA synthesis on chromatin. Thus, we have shown that the number of initiation sites using a mammalian enzyme is much less than that for a bacterial polymerase. Using calf thymus RNA polymerase B, we find 50,000 initiation sites for the whole calf thymus genome, and only 5000 sites on chromatin. Using artificial complexes of DNA and histone we are trying to determine which histones are responsible for this restriction. Finally we have begun to examine the chromatin from the slime mold Dictosteleum discoideum in order to use this as a simple model system.